Water bottom cable seismic survey cable and system

ABSTRACT

A seismic sensor cable is disclosed. The cable includes an outer jacket disposed on an exterior of the cable. The outer jacket excludes fluid from entering an interior of the cable. A reinforcing layer disposed within the outer jacket, which includes at least one electrical conductor disposed therein. An inner jacket is disposed within the reinforcing layer, and at least one electrical conductor disposed within an interior of the inner jacket. Some embodiments include at least one seismic sensor electrically coupled to the at least one electrical conductor disposed in the reinforcing layer In some embodiments a housing is disposed over the electrical coupling of the sensor to the conductor. The housing is molded from a polyurethane composition adapted to form a substantially interface-free bond with the cable jacket when the polyurethane cures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of, and claims priorityfrom, United States Nonprovisional patent application Ser. No.10/855,177 filed on May 27, 2004, the entirety of which is incorporatedherein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of marine seismic surveysystems. More particularly, the invention relates to marine seismicsystems deployed on the floor or bottom of a body of water.

2. Background Art

Marine seismic survey systems known in the art include so-called “oceanbottom cables” (OBCs). OBCs are deployed on the bottom of the ocean orother body of water, beneath which it is desired to determine thegeologic structure and rock characteristics. A typical OBC includes aplurality of seismic sensors at spaced apart locations along a cable.One or more seismic energy sources are towed in the water by one or morevessels, and are periodically actuated. Seismic signals detected by thesensors in the OBC are recorded, typically by a recording unit formingpart of the OBC.

The sensors in an OBC typically include motion sensors, such asgeophones or accelerometers. The motion sensors are typically packagedin such a way that the motion sensors are disposed along differentsensitive directions. “Sensitive direction” means the direction alongwhich the particular motion sensor is most sensitive to movement. As isknown in the art, typical motion sensors are primarily sensitive tomovement along a “principal axis” or principal direction, and aresubstantially insensitive to movement along any other direction. Havingsuch motion sensors provides the OBC with the capacity to detect seismicenergy propagating along a plurality of directions and to resolve thedirection of such movement.

Typical OBCs also include pressure sensors or pressure gradient sensors,such as hydrophones, at spaced apart locations along the cable.Hydrophones generate a signal proportional to pressure change in thesurrounding medium (the water). Having a pressure change signal relatedto seismic energy propagation, combined with motion signals alongdifferent sensitive directions, enables using many different techniquesknown in the art for interpretation of the signals so as to reduce theeffects of acquisition artifacts such as water bottom multiplereflections and water layer multiple reflections. The multiple directionmotion signals also enable interpretation of converted wave(compressional to shear) seismic signals, for determining properties ofthe subsurface earth formations such as amplitude versus offset (AVO)and directional anisotropy.

A number of “deghosting” and water layer effect attenuation techniquesare known in the art for use with OBCs. One such technique is describedin U.S. Pat. No. 4,486,865 issued to Ruehle. Pairs of detectors eachcomprise a geophone and a hydrophone. A filter is applied to the outputof at least one of the geophone or hydrophone in each pair so that thefrequency content of the filtered signal is adjusted. The adjustment tothe frequency content is such that when the filtered signal is combinedwith the signal from the other sensor, the ghost reflections cancel.

U.S. Pat. No. 5,621,700 issued to Moldovenu also discloses using atleast one pair of sensors in a method for attenuating ghosts and waterlayer reverberations.

U.S. Pat. No. 4,935,903 issued to Sanders et al. discloses a method forreducing the effects of water later reverberations which includesmeasuring pressure at vertically spaced apart depths, or by measuringpressure and particle motion using sensor pairs. The method includesenhancing primary reflection data for use in pre-stack processing byadding ghost data.

U.S. Pat. No. 4,979,150 discloses a method for marine seismicexploration in which output of substantially collocated hydrophones andgeophones are subjected to a scale factor. The collocated hydrophonesand geophones can be positioned at the sea floor or above the sea floor.

The foregoing description is intended to emphasize the potentialbenefits of seismic surveys acquired using OBCs. A limitation to usingOBCs is that it takes a substantial amount of time to deploy OBCs, andspecialized handling equipment is typically required to extend the OBCfrom a deployment vessel, and place the OBC on the water bottom. Afterdeployment, it is often necessary to determine the exact position on thesea floor at which each sensor in the OBC ultimately comes to rest onthe water bottom, because currents in the water, and viscous effects onthe various components of the OBC may cause some of the sensors to cometo rest at a different location than the location at the water surfaceof each sensor when the OBC was extended from the deployment vessel. Itis also necessary to retrieve the OBC to access a recording devicecoupled to the OBC in order to make use of the signals generated by eachof the sensors in the OBC. To survey a substantial geographic area usingOBCs thus requires a number of deployments and retrievals of the OBCsused in any survey operation.

The need for repeated deployment and retrieval of OBCs has made itnecessary for the various mechanical load handling components in atypical OBC to withstand repeated applications of axial stress along theOBC cable and along interconnecting devices that couple the variouscomponents of the OBC. It is also necessary for the various componentsof an OBC to withstand immersion in water, sometimes to substantialdepth (as much as 1,000 meters). While it is well known in the art howto form cables, sensor enclosures and interconnecting devices for OBCsto withstand environmental and operational stresses such as theforegoing, the devices known in the art are frequently heavy, cumbersomeand expensive to manufacture.

It is also known in the art to use modified versions of typical seismicsensing equipment intended for use on dry land, however, such modifiedland systems are typically suitable only for relatively shallow waterdepths (15 to 30 meters). There is a need for inexpensive tomanufacture, rugged OBCs that are suitable for use in greater waterdepths than modified land-based sensor systems, and that are easier todeploy and retrieve than typical OBCs.

SUMMARY OF THE INVENTION

One aspect of the invention is a seismic sensor cable. A seismic sensorcable according to this aspect of the invention includes an outer jacketdisposed on an exterior of the cable. The outer jacket is adapted toexclude fluid from entering an interior of the cable. A reinforcinglayer is disposed within the outer jacket. The reinforcing layerincludes at least one electrical conductor disposed therein. An innerjacket is disposed within the reinforcing layer. At least one electricalconductor is disposed within an interior of the inner jacket. In oneembodiment, the reinforcing layer is a fiber braid. In one embodiment,the outer jacket is formed from a polyurethane composition that forms asubstantially interface-free bond with uncured polyurethane upon curethereof.

Another aspect of the invention is a signal processing module for aseismic sensor system. A signal processing module according to thisaspect of the invention includes a housing having at least one connectortermination thereon. A circuit mounting frame is disposed within thehousing. The frame is adapted to sealingly engage an interior surface ofthe housing so as to define a chamber sealed from fluid enteringtherein. The module includes signal processing circuits mounted to theframe within the chamber.

In one embodiment, the housing comprises at least two connectorterminations. The two terminations include electrical connections to thesignal processing circuits. The signal processing circuits are adaptedto detect command signals originating from a recording unit operativelycoupled to the module from one of the at least two terminations. Thesignal processing circuits are further adapted to electrically couplethe other one of the at least two terminations to portions of the signalprocessing circuits adapted to receive signals from a seismic sensor.

Another aspect of the invention is a seismic data recording system. Asystem according to this aspect of the invention includes a recordingunit. A first signal processing module is operatively coupled to therecording unit. A first seismic sensor cable is operatively coupled atone end to the first signal processing unit, and is operatively coupledat its other end to a second signal processing unit. The first seismicsensor cable has at least one seismic sensor operatively coupledthereto. A second seismic sensor cable is operatively coupled to thesecond signal processing unit. The second seismic sensor cable also hasat least one seismic sensor operatively coupled thereto. The firstsignal processing unit and the second signal processing unit each haveat least two electrical terminations thereon. The first and secondsignal processing units each have circuits therein adapted to detectcommand signals originating from the recording unit. The circuits arealso adapted to selectively couple a data telemetry output to the one ofthe at least two terminations from which the command signals aredetected. The circuits are further adapted to selectively couple theother of the at least two terminations to portions of the circuitsadapted to receive signals from a seismic sensor.

Other aspects and advantages of the invention will be apparent from thedescription and claims that follow.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows principal components of one embodiment of an ocean bottomcable (OBC) seismic sensor and signal processing system according to thevarious aspects of the invention.

FIG. 2 shows internal construction of one embodiment of a pressuresealed connector and a signal processing module.

FIGS. 3A and 3B show one embodiment of a seismic sensor cable, in endview and side view, respectively.

FIG. 4 shows one embodiment of a sensor takeout coupled to the examplecable of FIG. 3.

FIG. 5 shows one embodiment of an hermaphroditic connector used tocouple the cable of FIG. 3 to a like or different sensor cable.

FIG. 6 shows one embodiment of sensor cables, additional sensor cables,seismic sensors and signal processing modules according to theinvention.

FIG. 7 shows a functional block diagram of one possible embodiment of asensor cable and a signal processing unit.

FIG. 8 shows an alternative embodiment of a sensor system.

FIG. 9 shows an alternative embodiment of a sensor system.

FIG. 10 shows an alternative embodiment of a signal processing module.

DETAILED DESCRIPTION

FIG. 1 shows principal components of an ocean bottom cable (OBC) seismicsensor and signal processing system (“system”) according to the variousaspects of the invention. The system 10 includes a signal processingmodule 18 that is coupled at one end to a seismic sensor cable 12.Various electronic circuits (explained below with reference to FIG. 2)inside the signal processing module 18 detect electrical signalsgenerated by one or more seismic sensors, shown generally at 16, andconvert the electrical signals into a suitable form for transmissionand/or recording. The signal processing module 18 in the presentembodiment may also be coupled at its other end to a data communicationcable 13 having electrical conductors therein (not shown separately) fortransferring the processed signals to a recording unit (not shown inFIG. 1) or to another (not shown in FIG. 1) signal processing modulesimilar in configuration to the signal processing module 18 shown inFIG. 1. In other embodiments, the signal processing module 18 may becoupled at its other end to another seismic sensor cable (not shown inFIG. 1), similar in configuration to the seismic sensor cable 12 shownin FIG. 1.

The signal processing module 18 is coupled to the sensor cable 12 and tothe data communication cable 13 (or to another sensor cable) by apressure-sealed electrical/mechanical connector (“sealed connector”) 20.One such sealed connector 20 is disposed at one end of the datacommunication cable 13. Another sealed connector is disposed at one endof the seismic sensor cable 12, as shown in FIG. 1. The internalstructure of the sealed connector 20 will be further explained belowwith reference to FIG. 2. Internal structure of the signal processingmodule 18 will also be explained with reference to FIG. 2.

The seismic sensor cable 12 includes one or more sensor “takeouts” 14.Each sensor takeout 14 is a sealed, pressure resistant enclosure affixedto the exterior of the sensor cable 12. The takeout 14 includes withinits interior electrical connections between electrical conductors(explained with reference to FIG. 4) disposed in an outer reinforcingbraid (not shown in FIG. 1) in the cable 12 and one or more of theseismic sensors 16. As will be further explained, the takeout 14 alsoprovides strain relief to the connection between the sensor 16 and thecable 12.

The seismic sensor 16 can be a single component geophone, a multiplecomponent (typically three orthogonal components) geophone, ahydrophone, an accelerometer, combination hydrophone/geophone, or anyother device known in the art for detecting seismic signals. Theconstruction of the sensor cable 12 will be explained in more detailwith reference to FIGS. 3A and 3B. The construction of the sensortakeout 14 will be explained in more detail with reference to FIG. 4.While the embodiment of the sensor cable 12 shown in FIG. 1 includes onesensor takeout 14, having one seismic sensor coupled at the takeout 14,other embodiments of a seismic sensor cable according to the inventionmay include more than one sensor takeout 14. Still other embodiments ofa seismic sensor cable may include more than one seismic sensor coupledto the cable at each sensor takeout 14, or at any one or more suchtakeouts along a sensor cable.

The end of the seismic sensor cable 12 opposite to the end coupled tothe signal processing module 18 may be terminated with an hermaphroditicconnector 22. A like-configured hermaphroditic connector 22 is disposedat one end of a sensor extension cable 15. The two like-configuredhermaphroditic connectors 22 are mechanically coupled to each other byan internally threaded coupling or ring 24 that engages correspondingthreads on the exterior of each of the joined hermaphroditic connectors22. In the present embodiment, the sensor extension cable 15 may or maynot include additional seismic sensors (not shown in FIG. 1).Preferably, the sensor extension cable 15 includes a like-configuredhermaphroditic connector 22 at both its ends, such that either end ofthe sensor extension cable 15 may be coupled to the hermaphroditicconnector 22 on the end of the seismic sensor cable 12. It is alsopreferable to include one threaded coupling ring 24 for each end of thesensor extension cable 15. By having an hermaphroditic connector 22 andan associated threaded coupling ring 24 at each end of the sensorextension cable 15, mishandling of the sensor extension cable 15 (i.e.,deploying the cable 15 backward) avoids having an incorrect cabletermination arranged to mate with the corresponding hermaphroditicconnector 22 on the end of the seismic sensor cable 12. Similarly,having an hermaphroditic connector at each end of the sensor extensioncable 15 avoid having an incorrect termination arranged to mate twosensor extension cables end-to end. Construction of the hermaphroditicconnector 22 will be explained in more detail with reference to FIG. 5.

FIG. 2 shows one embodiment of the sealed connector 20 used to couplethe sensor cable 12 to the signal processing module 18, and showsinternal construction of the signal processing module 18 in more detail.In the present embodiment, the sealed connector 20 includes an outershell 20F that can be molded from plastic. The shell 20F is preferablymolded to as to define an interior space 20E. In the present embodiment,the plastic from which the shell 20F is molded is a fiber-filledpolyurethane compound sold under the trade name HYDEX by the A. L. HydeCompany, Grenloch, N.J. Other types of plastic may be used in otherembodiments, however one important attribute of the plastic is that itform a substantially interface-free bond with uncured plastic used tofill an interior space in the shell 20F during assembly of the connector20. Attributes of such plastic compounds will be further explained. Theshell 20F may also be molded to include suitable locations or grooves20D for o-rings or similar sealing elements in order to form afluid-tight seal against the inner surface of a housing 18A, when theconnector 20 is engaged with the housing 18A. The housing 18A encloseselectronic components of the signal processing module 18, which will befurther explained. The shell 20F also may be molded to include a lockingring groove 20C to transmit axial load between the signal processingmodule housing 18A and the shell 20F.

The axial end of the shell 20F that mates with the housing 18A mayinclude a recessed opening adapted to mate with one side of anelectrical contact supporting wafer 20B. The electrical contactsupporting wafer 20B is also preferably made from molded plastic,preferably the HYDEX compound referred to previously. The electricalcontact supporting wafer 20B is molded to include electrical contacts20A within the structure of the support wafer 20B. The electricalcontacts 20A are arranged to enable mating with corresponding electricalcontacts 18F disposed in the signal processing module 18. The electricalcontacts 20A are preferably molded into the wafer 20B during fabricationof the wafer 20B so as to form a substantially impenetrable barrier tomovement of fluid through the wafer 20B at the location of theelectrical contacts 20A. Having the electrical contacts 20A thus moldedinto the wafer 20B can substantially prevent fluid entry into theinterior of the connector 20 in the event the connector 20 becomesdisengaged from the module 18.

In the present embodiment, the electrical contact supporting wafer 20Bis preferably molded such that each of the electrical contacts 20Arecessed into a small, open-ended cylindrical tube (not shown separatelyfor clarity) which seals radially against a corresponding substantiallycylindrical tube (not shown for clarity) enclosing each of theelectrical contacts 18F in the signal processing module 18. Byconfiguring the contact wafer 20B and the corresponding electricalcontact supporting structure in the module 18 to have such sealingtubes, the electrical contacts 20A and 18F will be sealed against fluidintrusion even if other seal elements in the connector/housingarrangement (for example, o-rings in grooves 20C) fail.

The shell 20F preferably is molded to include a tapered exteriordiameter, called a “bend radius transition” and shown at 20D, such thatthe bending stiffness of the connector 20 having the cable 12 thereingradually changes from relatively stiff at the joined end of theconnector 20, to the bending stiffness of the cable 12, thus avoidingapplying undue bending stress to the cable 12 where it joins theconnector 20. Other embodiments may include a separately molded bendradius transition 20D molded from a softer (or more flexible) plasticthan the plastic used to form the shell. Such separately molded bendradius transitions may be molded onto the shell 20F after assembly ofthe connector 20.

The axial end of the electrical contact support wafer 20B that isadapted to mate with the electrical contact portion of the signalprocessing module 18 includes a tubular extension having seal groove 20Kin its exterior surface, suitable to retain an o-ring or similar sealelement. The seal element in the groove 20K engages the interior surfaceof the electrical contact portion of the signal processing module 18 soas to exclude fluid entry into the electrical contact area even if theother sealing devices (such as the o-rings in grooves 20D) fail.

To assemble the connector 20 to the seismic sensor cable 12, electricalconductors 12C and a fiber reinforcing layer 12A in the cable areexposed by appropriate stripping of the cable outer jacket 12D and cableinner jacket 12B. The electrical conductors 12C are soldered, crimped orotherwise fastened to the electrical contacts 20A in the contact supportwafer 20B. The contact support wafer 20B is then placed into the matingarea at the axial end the shell 20F. Finally, the interior 20E is filledwith uncured plastic such as polyurethane, preferably a compound soldunder the trade name SMOOTH-CAST 320 by Smooth-On, Easton, Pa.Irrespective of the actual polyurethane or other plastic used to fillthe interior 20E, an important characteristic of the plastic used tofill the interior 20E is that when the plastic solidifies or sets, theplastic forms what is in effect a molecular bond with the shell 20F.Advantageously, using such a polyurethane or other plastic compositionhaving such bonding attributes transfers substantially all the axialload applied to the connector 20 equally to each individual fiber in thefiber layer 12A. Having such equalized axial load transfer can providethe maximum possible axial strength of the connector 20 as assembled tothe cable 12. Additionally, using polyurethane having the describedproperties can substantially prevent fluid intrusion into the cable 12or the interior 20E of the connector 20, even when the connector 20 isdisengaged from the signal processing module 18, or the sealing elements(for example, O-rings in the grooves 20C, 20K) fail.

The connector 20 could be molded as a single component having all thepreviously explained geometric features therein, merely by locating theelectrical contacts 20A in suitable positions, providing a suitablyshaped mold and filling the mold with a suitable plastic compound. Apossible advantage of assembling the connector as described above,however, is that the connector 20 may be entirely replaced on a seismicservice vessel or in any other facility lacking suitable plastic moldingfixtures using the described premolded shell 20F and the describedpremolded electrical contact support wafer 20B.

Still referring to FIG. 2, the housing 18A for the signal processingmodule 18 is preferably made from titanium, stainless steel or similarhigh strength material. The material is preferably resistant to crevicecorrosion and stress crack corrosion. The housing 18A may besubstantially cylindrical in interior and exterior shape. The housing18A includes on its exterior surface, at its axial ends, threads 18Ethat are arranged to mate to corresponding threads on the interiorsurface of a locking element retaining ring 19. When the connector shell20F is mated to the housing 18A, a locking element (not shown forclarity) is inserted through lock openings 18D in the housing 18A. Thelocking element (not shown) also engages the lock ring groove 20C formedin the shell 20F. The locking element (not shown) performs the functionof axially retaining the connector shell 20F in the housing 18A. Thelocking element retaining ring 19 is then threaded onto the threads 18Ein the housing 18A so as to cover the locking element opening 18D, thusretaining the locking element (not shown) within the locking elementopening 18D.

A circuit board mounting frame 18B is inserted into the housing 18A. Thecircuit board mounting frame 18B has fastened thereto a circuit board18G that includes the active circuitry of the signal processing module18. One possible embodiment of circuitry will be explained withreference to FIG. 7. Another embodiment will be explained with referenceto FIG. 10. The number of boards, and the particular circuits disposedon any embodiment of the circuit board 18G are a matter of discretionfor the system designer, and will depend on the intended arrangement ofseismic sensors (such as sensor 16 in FIG. 1), among otherconsiderations. The frame 18B is preferably configured such that thecircuit board 18G may be easily removed from the frame 18B when theframe 18B is removed from the housing 18A. Axial ends of the frame 18Bpreferably include a recess to accept an electrical contact supportwafer 18J similar in configuration to the contact support wafer 20Bdisposed in the mating end of the connector shell 20F. Electricalcontacts 18F may be molded into the wafer 18J during manufacture,similar to the wafer of the connector 20. Alternatively, the electricalcontacts 18F may be molded directly into the frame 18B duringmanufacture. The frame 18B may be molded from plastic, preferably apolyurethane such as the previously described HYDEX compound.

The axial ends of the frame 18B also preferably include sealing portions18H, which in the present embodiment comprise substantially cylindricalportions, sized to fit snugly inside the interior surface of the housing18A, and include grooves 18C for o-rings or similar sealing elementtherein. The purpose of the sealing portions 18H, in combination withsuitable sealing elements, is to exclude fluid from entering theinterior of the housing 18A, thus damaging the circuit board 18G, in theevent one of the connectors 20 becomes uncoupled from the housing 18Aduring use, or in the event the sealing elements (for example, o-ringsin grooves 20D) on the connector 20 fail.

One embodiment of the seismic sensor cable 12 is shown in end view inFIG. 3A and in side cut-away view in FIG. 3B. The sensor cable 12includes an outer sheath or jacket 12D, preferably made frompolyurethane. The outer jacket 12D is intended to exclude fluid from theinterior of the cable 12. Preferably, the polyurethane composition usedto form the jacket 12D will provide what is in effect a molecular bondwhen placed in contact with uncured polyurethane, as previouslyexplained with respect to assembly of the sealed connector (20 in FIG.2). Other materials may be used to form the jacket 12D provided that thematerial selected can for a suitable bond with the material used to fillthe interior (20E in FIG. 2) of the shell (20F in FIG. 2).

A woven fiber reinforcing braid 12A is disposed inside the jacket 12D.The reinforcing braid 12A may be formed from glass fiber, combinationglass/graphite fiber, polymer fiber or any similar material orcombination thereof known in the art for reinforcing a cable or tube.The reinforcing braid 12A should have a fiber weave, fiber size andfiber material selected to provide the cable 12 with an axial breakingstrength preferably equal to about twice the expected axial loading onthe cable 12 during use. In one embodiment, the intended axial load onthe cable during use is at most about 1,000 lbs. In the presentembodiment, the fiber braid 12A should provide the cable with an axialbreaking strength of about 2,000 lbs.

In the present embodiment, some of the individual fibers or groups offibers in the braid 12A may be substituted with small-gauge individualinsulated electrical conductors or twisted pairs of insulated electricalconductors, shown at 12C. The insulated electrical conductor pairs 12Cin the present embodiment are coupled at selected locations along thecable 12 to one or more seismic sensors (16 in FIG. 1), as previouslyexplained, at a sensor takeout (14 in FIG. 1).

Disposed inside the braid 12A is an inner jacket 12B. The inner jacket12B provides electrical insulation and a fluid-tight enclosure for acentral electrical conductor 12E. The central conductor 12E is shown asa single conductor, however other embodiments of a sensor cable mayinclude a plurality of conductors disposed inside the inner jacket. Theinner jacket 12B in the present embodiment may be made from plastic soldunder the trade name TEFLON by E. I. du Pont de Nemours & Co.,Wilmington, Del. Other suitable materials for the inner jacket 12B areknown in the art, including polyurethane.

An important purpose for the inner jacket 12B in some embodiments of thesensor cable 12 is to exclude fluid from the central conductor 12E inthe event the outer jacket 12D becomes damaged to an extent that fluidis admitted through the outer jacket 12B. In one embodiment, as will beexplained below with reference to FIG. 6, the central conductor(s) 12Emay be used to carry electrical power and data between a centralrecording unit (not shown in FIGS. 3A and 3B) and one or more of thesignal processing units (18 in FIG. 1). As previously explained, theinsulated electrical conductors 12C disposed within the braid 12A may beused to conduct signals from one or more seismic sensors (16 in FIG. 1)disposed along the cable 12 to one of the signal processing units (18 inFIG. 1). A cable configured as shown in FIGS. 3A and 3B may thus be lesssusceptible to failure of power transmission and data communication (orany other function requiring unimpaired electrical insulation andcontinuity) along the central conductor(s) 12E in the event of failureof the outer jacket 12B. An OBC system configured as shown in FIG. 1 isthus more likely to maintain successful operation of a substantialportion of the OBC system even in the event of failure of a portionthereof by fluid intrusion into one or more of the sensor cables 12.

As previously stated, the embodiment of the cable 12 shown in FIGS. 3Aand 3B includes one centrally positioned electrical conductor 12E,however, it should be understood that any number of conductors disposedinside the inner jacket 12B may also be used. The number of and size ofsuch electrical conductors in any particular embodiment of a sensorcable will depend on application-specific criteria, including the typeof data telemetry used between signal processing units and/or between acentral recording unit, whether and how much electrical power istransmitted along the central conductor(s), and the length of thevarious individual sensor cables, among other criteria. Accordingly, thenumber of centrally positioned electrical conductors is not intended tolimit the scope of the invention.

Electrical cable configured substantially as shown in FIGS. 3A and 3Bmay be used, in some embodiments, to form seismic sensor cables (12 inFIG. 1) by including a sealed connector, such as 20 in FIG. 1 (forcoupling to a signal processing module) at one end and an hermaphroditicconnector (as will be explained with reference to FIG. 5) at the otherend. The electrical cable of FIGS. 3A and 3B may also be used to formsensor extension cables (15 in FIG. 1) by including an hermaphroditicconnector at each end.

One embodiment of a sensor takeout is shown in FIG. 4. The sensortakeout 14 may include a molded polyurethane outer housing 23. The outerhousing 23 may be molded over the outer jacket 12D of the sensor cable12. At a location where one or more seismic sensors 16 are to be coupledto the seismic sensor cable 12, a selected length of the outer jacket12D may be removed from the seismic sensor cable 12, (or, alternatively,small openings may be made in the outer jacket 12D) and one or more ofthe twisted pairs of insulated conductors 12C is extracted from thefiber braid 12A. The conductors 12C are then electrically coupled toelectrical leads 16A the seismic sensor 16. The leads 16A aremechanically affixed to the outer jacket 12D to relieve strain, usinghoop wraps 21 that may be fiber, plastic or other suitable material. Insome embodiments, the leads 16A may themselves be wrapped around thecable 12 to provide strain relief. Preferably, a mounting block 17 isdisposed between the leads 16A and the outer jacket 12D to reducecrushing or pinching of the outer jacket 12D and leads 16A. Theelectrical connections between the leads 16A and the conductors 12C arepreferably coated, prior to molding the outer housing 23, with a solventevaporation-curing rubberized plastic insulating compound (not shown),such as one sold under the trade name PLASTI DIP by Plasti DipInternational, Blaine, Minn. Coating may be performed by immersion,spraying or brushing. The insulating compound (not shown) helps ensureelectrical insulation over the electrical connections between theconductors 12C and the leads 16A in the event of fluid intrusion intothe outer housing 23.

After assembly of the leads 16A to the cable 12, as explained above, theouter housing 23 may then be molded over the cable 12. In the presentembodiment, each of the sensor leads 16A includes a pressure-sealedelectrical/mechanical connector 16B to enable changing sensors 16, ifsensor failure should occur, without the need to rebuild the sensortakeout 14. Other embodiments may omit the connector 16B. Suitableconnectors for the purpose of making electrical and mechanicalconnection between the sensor 16 and the sensor takeout 14 are wellknown in the art.

The sensor takeout 14 shown in FIG. 4 includes two individual seismicsensors 16, however, the number of sensors used in other embodiments ofa takeout may be more or fewer. The sensor takeout shown in FIG. 4 hasbeen successfully tested to resist fluid intrusion at an external fluidpressure equivalent to a water depth of 300 meters.

One embodiment of an hermaphroditic connector as used on the sensorcable 12 and on sensor extension cables (15 in FIG. 1) is shown in FIG.5. The connector 22 in the present embodiment includes a molded outershell 22H which may be formed from polyurethane, similar to the HYDEXcompound used for the connector shell (20F in FIG. 2) shown in FIG. 2.The hermaphroditic connector shell 22H includes an interior bore orpassage 22A that terminates in a receptacle for receiving an electricalcontact support wafer 22B. The interior passage 22A of the shell 22H ispreferably no smaller than needed to enable free passage of the sensorcable 12. The shell 22H includes a threaded portion 22C molded into itsexterior surface at one axial end, for coupling to a corresponding oneof the hermaphroditic connectors, as will be further explained. Theshell 22H preferably includes a bend radius transition 22J to avoidexcessive bending stress on the sensor cable 12 during handling andoperation.

The support wafer 22B is preferably molded from the same or similarpolyurethane material from which the shell 22H is molded. Electricalcontacts are appropriately arranged inside male 22E and female 22Dcontact sealing tubes. The contacts are molded into the body of thewafer 22B during manufacture. The contact sealing tubes 22D, 22E,preferably include integrally molded seal rings 22G, 22F respectively,such that when the sealing tubes 22D, 22E are mated with correspondingsealing tubes on another one of the hermaphroditic connectors, thesealing tubes 22D, 22E are sealed against intrusion of fluid.

During assembly of the connector 22, electrical conductors 12C in thecable 12 are coupled to the electrical contacts in the sealing tubes22E, 22D, such as by soldering, crimping or other method known in theart. The contact wafer 22B is then placed in its receptacle at the axialend of the shell 22H. The interior of the shell 22A is then filled withuncured polyurethane, such as the previously described SMOOTH-CAST 320compound. When cured, the polyurethane compound forms what isessentially a molecular bond to the shell 22H, to the cable outer jacket12D and to the contact wafer 22B. Having such a bond between the variouscomponents of the interior of the connector 22 minimizes the possibilityof fluid intrusion into the interior of the connector 22 and the cable12, even in the event the connector 22 becomes disengaged from itsmating hermaphroditic connector.

As previously explained with respect to FIG. 1, axial load istransferred between mated ones of the hermaphroditic connectors 22 by athreaded coupling ring 24 threaded over the threaded portions 22C formedinto each mating connector shell 22H. Also as previously explained, itis desirable to have a threaded ring 24 disposed at each end of sensorextension cables (15 in FIG. 1) which have an hermaphroditic connectorat both ends. The hermaphroditic connector shown in FIG. 4 has beensuccessfully tested to resist an externally applied fluid pressureequivalent to a water depth of 300 meters.

One example arrangement of the various components of a system accordingto the invention is shown in FIG. 6. A first sensor extension cableincluding hermaphroditic connectors at each end is shown at 115. Thefirst sensor extension cable 115 includes two sensor takeouts 14. Eachtakeout 14 includes two seismic sensors 16. One end of the first sensorextension cable 115 includes a plug 27 in the hermaphroditic connector22 to seal the connector 22 against fluid entry. The first sensorextension cable 115 is coupled at its other end to one end of asimilarly formed, second sensor extension cable 215 using a threadedring 24 to threadedly connect two of the hermaphroditic connectors 22.The second sensor extension cable 215 includes two sensor takeouts 14each having two seismic sensors 16 coupled thereto.

The second sensor extension cable 215 is coupled at its other end to oneend of a first sensor cable 112. The first sensor cable 112 may beformed with an hermaphroditic connector 22 at one end, and a sealedconnector 20, as described with reference to FIG. 2, at the other end.The first sensor cable 112 includes two sensor takeouts 14 each havingtwo seismic sensors 16 coupled thereto.

The other end of the first sensor cable 112 is coupled to a first signalprocessing module 118. The first signal processing module 118 may beformed as explained above with reference to FIG. 2. In the arrangementshown in FIG. 6, the circuits (not shown separately) in the first signalprocessing module 118 interrogate the various sensors “downstream” (fromthe first module 118 to the end of the first additional sensor cable115) from the first module 118 and convert the signals thus interrogatedinto a form suitable for transmission “upstream” through a second sensorcable 212, through a third sensor extension cable 315, through a thirdsensor cable 312, to a second signal processing module 218 for inclusionin signal telemetry sent from the second signal processing module,through a data communication cable 13 to a recording unit 50. Aspreviously explained with reference to FIGS. 3A and 3B, transmission ofprocessed signals upstream from the first module 118 to the secondmodule 218 is preferably along one or more centrally positionedconductors (not shown separately) in each of the third sensor extensioncable 315 and second and third sensor cables, 212, 312, respectively, sothat in the event of failure of the outer jacket on any of these cables215, 212, 312, or failure by fluid intrusion at any of the sensortakeouts 14, signal communication with the still-operative sensors 16may continue, and data communication upstream may continue. In someembodiments, command signals may be transmitted from the recording unit50 to each of the modules 118, 218 to control various aspects of thesignal processing, particularly time indexing. Such command signals arealso preferably transmitted along the central conductor(s) to reduce thechance of communication failure.

The second 212 and third 312 sensor cables are similar in configurationto the first sensor cable 112. The third sensor extension cable 315 issimilar in configuration to the first 115 and second 215 sensorextension cables. The data communication cable 13 preferably onlyincludes a central conductor as explained with reference to FIGS. 3A and3B, deleting the twisted pairs of conductors disposed in the fiberreinforcing braid, because no sensor takeouts are included in the datacommunication cable 13. The recording unit 50 may be a conventionalrecording unit known in the art for use with OBC systems.

A functional block diagram of one possible embodiment of a sensor cable(12 in FIG. 6) and a signal processing module (18 in FIG. 6) is shown inFIG. 7. The sensor cable 12 in the present embodiment includes eight ofthe twisted pairs of exterior electrical conductors (12C in FIGS. 3A and3B) for conducting signals from seismic sensors 16 connected to thesensor cable 12 to the signal processing module 18. Each sensor 16 iscoupled to its respective twisted pair (shown as single lines) S1, S2,S3, S4 by a sensor takeout 14 made as explained earlier with respect toFIG. 4. The other four twisted conductor pairs, indicated by lines T1,T2, T3, T4 are “through” conductors for carrying analog signals such asmay be generated by sensors in another sensor cable. The twisted pairsof conductors T1-T4 and S1-S4 terminate in a sealed connector 20 at oneend of the sensor cable 12. The sealed connector 20 may be formedsubstantially as explained with respect to FIG. 2. The sealed connector20 mates with a corresponding connector (not shown separately) on thesignal processing module 18. The sensor cable 12 may include a datacommunication conductor COM1, disposed within the inner cable jacket (asexplained with reference to FIGS. 3A and 3B). In the embodiment of FIG.7, the data communication conductor COM1 may carry telemetry and/orelectrical power depending on the particular connections to each end ofthe sensor cable 12.

The signal processing module 18 in the present embodiment includeswithin its circuits a telemetry transceiver 105 for detecting commandsignals from the recording unit (50 in FIG. 6). The telemetrytransceiver 105 preferably also performs the functions of a systemcontroller. Programmable integrated circuits such as ASICs (applicationspecific integrated circuits) are known in the art for performing thefunctions of the transceiver/controller as described herein. On powerup, the transceiver 105 may be programmed to detect which one of thecommunications ports COM2A or COM2B includes command signals from therecording unit (50 in FIG. 6). As can be inferred from the descriptionwith reference to FIG. 2, the communications ports COM2A, COM2B formpart of the electrical and mechanical connectors arranged to mate withsealed connectors 20 on either the sensor cable 12 or the datacommunication cable (13 in FIG. 6). Depending on which communicationsport COM2A or COM2B is determined to be the source for command signals,the transceiver 105 operates a telemetry port switch 103 to coupletelemetry signal output to the same communications port. The othercommunications port COM2B or COM2A will be selected to receive“downstream” telemetry from any other signal processing units. One sucharrangement of downstream signal processing unit has been explained withreference to FIG. 6. Depending on which communications port isdetermined to be the “upstream” port, an analog signal input switch 104selects which connector will be a source for analog signals from theseismic sensors 16 located “downstream” from the signal processing unit18. In FIG. 7 these connectors are indicated by ASIG1 and ASIG2. Itshould be noted that the electrical contacts for the communicationsports COM2A and COM2B in the present embodiment are disposed within eachof the same electrical/mechanical connectors as contains the analogsignal ports ASIG1 and ASIG2.

Depending on which analog signal ports ASIG1 or ASIG2 is selected by theanalog signal input switch 104, the selected analog signal input may becoupled to a multiplexer (MUX) 102 to convert parallel analog inputsfrom the selected analog signal port to serial input. The serial inputmay then be digitized in an analog to digital converter (ADC) 101 forfiltering (such as by a finite impulse response filter) and buffering inthe transceiver 105. Buffered signal data may be transmitted upstream onthe selected data communication port when commanded by the recordingunit (50 in FIG. 6) or in a preprogrammed telemetry format. The actualtelemetry format is a matter of discretion for the system designer andis not intended to limit the scope of the invention.

The “downstream” data communication port, COM2A or COM2B depending onthe command signal detection as previously explained, may beperiodically interrogated by the transceiver 105 for the presence ofdata being communicated upstream by another signal processing module(not shown in FIG. 7).

In some embodiments, performance of the sensor system may be improved bylimiting the number of analog sensor signals with respect to the numberof signal processing modules. FIG. 8 shows an alternative embodiment ofa seismic sensor system according to the invention that may provide suchimproved performance. The system includes sensor cables 12 that may bemade substantially as explained with reference to FIGS. 2, 3A, 3B, 4 and5. Each of the sensor cables 12 in the present embodiment of the systemincludes one sensor takeout 14. Each sensor takeout 14 on each sensorcable 12 has one seismic sensor 16 coupled thereto. One end of eachsensor cable 12 is terminated in a sealed connected 20 of the type (asexplained above with reference to FIG. 2) adapted to connect to a signalprocessing module, shown at 318 in FIG. 8. One arrangement of internalcomponents of the signal processing modules 318 for the presentembodiment will be explained below with reference to FIG. 10. The otherend of each sensor cable 12 terminates in an hermaphroditic connector22, which may be made substantially as explained with reference to FIG.5. Two of the signal processing modules 318 may be directlyinterconnected to each other by a sensor interconnect cable 312. Thesensor interconnect cable 312 may be made substantially as one of thesensor cables 12, and includes two sensor takeouts 14, each with oneassociated seismic sensor 16. The sensor interconnect cable 312 in thepresent embodiment is terminated at each end by one of the sealedconnectors 20, so as to be able to be coupled at each end to a signalprocessing module 318.

In the example system shown in FIG. 8, one end of one of the sensorcables 12 is terminated with a plug 27 coupled to the hermaphroditicconnector 22 at the end of one of the sensor cables 12. The other end ofthe one sensor cable 12 is connected to one end of a signal processingmodule 318. The signal processing module 318 is connected at its otherend to one end of the sensor interconnect cable 312. The other end ofthe sensor interconnect cable 312 is connected to one end of a secondsignal processing module 318. The other end of the second signalprocessing module 318 is connected to one end of a second sensor cable12. The other end of the second sensor cable 12 is terminated in anhermaphroditic connector 22. The system shown in FIG. 8 may itself becoupled to another such system at its “upstream” end (the open end ofthe hermaphroditic connector 22 on the end of the second sensor cable12). Some embodiments may include a plurality of systems as shown inFIG. 8 coupled end-to-end, terminating in a recording unit (such asshown at 50 in FIG. 6). The system shown in FIG. 8 may itself terminatein a recording unit (such as shown at 50 in FIG. 6). Irrespective of theexact number of systems coupled end to end, it is contemplated thatelectrical power and digitized signals are to be communicated alonginner conductors (12E in FIG. 3A) between the recording unit and one ormore signal processing modules. Analog signals from the various sensors16 are conducted from the sensors 16 to one of the signal processingmodules over the twisted pairs of conductors (12C in FIG. 3A) disposedin the fiber reinforcing braid (12A in FIG. 3A). The system shown inFIG. 8 thus may have the advantages explained with respect to the systemof FIG. 6, namely that in the event an outer jacket on one or more ofthe sensor cables 12 becomes damaged, and fluid intrudes past the outerjacket, power and digitized signals may continue to be communicatedbetween the recording unit and the various signals processing modulesbecause the inner jacket would continue to exclude fluid from enteringthe inner conductor(s).

An alternative arrangement including two seismic sensors on each sensortakeout is shown in FIG. 9. The system in FIG. 20 includes two seismicsensor cables 12, which may be made substantially as explained withreference to FIGS. 2, 3A, 3B, 4 and 5. Each sensor cable 12 isterminated with a sealed connector 20 at one end and an hermaphroditicconnector 22 at the other end. Each sensor cable 12 includes one takeout14, each of which is coupled to two seismic sensors 16. Each sensorcable 12 is coupled at the sealed connector 20 end to one end of asignal processing module 318. One end of the system shown in FIG. 9includes a plug 27 in the open end of one of the hermaphroditicconnectors 22. The other end of the system, terminated by openhermaphroditic connector 22 may be connected to another system“upstream” thereof, or may terminate in a recording unit (such as shownat 50 in FIG. 6). The system shown in FIG. 9 includes a moduleinterconnect cable, shown at 213. The module interconnect cable 213 isterminated at each end by a sealed connector. The module interconnectcable 213 may be electrically and mechanically configured similar to thepreviously described data communication cable (13 in FIG. 6) and isadapted, in the present embodiment, to carry only electrical power anddigitized signals between signal processing modules 318, and thereforehas no sensor takeouts or electrical conductors disposed within thefiber braid (12A in FIG. 3B).

One example of a signal processing module suitable for the systems shownin FIGS. 8 and 9 is shown in block diagram form in FIG. 10. The module318 in this embodiment includes two substantially identical circuitboards 218G disposed in the frame 18B in opposed directions, such thatirrespective of which end of the module 318 is connected to a particularcable (such as interconnect cable 213 in FIG. 9 or sensor cable 12 inFIG. 9, for example) the circuits connected to that cable will be thesame. Each board 218G includes a central processor (CPU) 202A which,among other functions, controls operation of Tx/Rx switches 205 andamplifier/ADC units 201. As in the previously described embodiments ofthe signal processing module, detection of command signals by the CPU202A will establish which end of the module 318 is coupled to the“upstream” part of the system (in the direction of the recording unit),and thus in which direction digitized signals are to be received fromdownstream modules, and to where digitized signals are to be transmittedfor recording or retransmission by another signal processing module,depending on the configuration used. Command signals may also be used toinstruct the CPU 202A to detect analog signals from particular ones ofvarious analog signal input terminals S1, S2.

Each circuit board 218G preferably has two sets of analog signalterminals S1, S2, and two sets of power/telemetry input/output terminalsT1-T4. In the present embodiment, connector terminations are arrangedsuch that one set of analog signal terminals and one set of digitalsignal terminals can mate with corresponding terminals in one sealedconnector (20 in FIG. 2) coupled to one end of the module 318, while atthe other end of the module 318, a similar set of analog signalterminals and a similar set of digital signal terminals can mate withcorresponding terminals in another sealed connector (20 in FIG. 2)coupled to the other end of the module 318. Digital signal communicationand power transfer between the two boards 218G in the module 318 aremade, in the present embodiment, by an internal jumper 218J. When usedin the embodiment of the system shown in FIG. 8, analog signals areinput to each end of the module 318. When used in the system embodimentof FIG. 9, analog signals are input only at one end of the module 318.

The various aspects of the invention provide components for an OBCsystem that are inexpensive to manufacture, deploy efficiently, and aremore resistant to fluid intrusion and associated failure than aresimilar systems known in the art.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A signal processing module for a seismic sensor system, comprising: ahousing having at least one connector termination thereon; a circuitmounting frame disposed within the housing, the frame adapted tosealingly engage in interior surface of the housing so define a chambersealed from fluid entering therein; and signal processing circuitsmounted to the frame within the chamber.
 2. The module as defined inclaim 1 wherein the circuits are adapted to receive signals from atleast one seismic sensor and to perform at least one of: formatting thesensor signals for telemetry, buffering the signals for transmissiontransmitting formatted signals to another signal processing unit andtransmitting the formatted signals to a recording unit.
 3. The module asdefined in claim 1 wherein the housing comprises at least two connectorterminations, the at least two terminations including electricalconnections to the signal processing circuits, the signal processingcircuits adapted to detect command signals originating from a recordingunit operatively coupled to the module from one of the at least twoterminations, the signal processing circuits adapted to electricallycouple the other one of the at least two terminations to portions of thesignal processing circuits adapted to receive signals from a seismicsensor.
 4. The module as defined in claim 3 wherein the signalprocessing circuits are adapted to selectively electrically couple atelemetry output thereof to the one of the terminations from which thecommand signals are detected.
 5. The module as defined in claim 4wherein the signal processing circuits are adapted to receive and formatfor inclusion in the telemetry output signals detected at a telemetryinput, the telemetry input selectively coupled by the signal processingcircuits to the other one of the at least two terminations.